1. Field of the Invention
The invention relates to organosilyl-functionalized particles and also to processes for preparing them using methylene-spaced alkoxysilanes.
2. Description of the Related Art
The term “organosilyl-functionalized particles” comprehends all particles having such functionalization, the cores of said particles being composed of metal oxides, metal mixed oxides, silicon oxide, silicon-metal mixed oxides and/or three-dimensionally crosslinked organosiloxane compounds (organopolysiloxane resins).
The application of conventional organopolysiloxane resins lies preferably in their use as binders, principally in coatings or in electrical insulation applications. Processes for preparing organopolysiloxane resins have been known for a relatively long time and are described for example in DE 198 57 348 A. Organopolysiloxane resins are prepared by hydrolyzing chlorosilanes or alkoxysilanes and then condensing the products to polysiloxanes. The direct reaction of chlorosilanes with water leads very readily to gelling. For this reason, partial alkoxylation prior to hydrolysis has proven particularly appropriate.
Where the preparation starts from alkoxysilanes, it is necessary to add hydrolysis catalysts, hydrochloric acid in the simplest case. In spite of this the hydrolysis of alkoxy groups is usually not quantitative, nor does the condensation reaction proceed to completion during resin preparation, with the consequence that fractions of hydroxyl and alkoxy groups remain in the resin. The residual content of these groups in the primary resins can be reduced, and the molecular weight increased, by subsequently employing condensation catalysts such as metal hydroxides, for example.
For the preparation of functionalized organopolysiloxane resins, EP 759 457 A, for example, cohydrolyzes functionalized organosilyl compounds with chlorosilanes or alkoxysilane building blocks; particularly in the case of hydrolysis starting from alkoxysilanes, hydroxyl groups remain in the organopolysiloxane resin. These groups can frequently not be removed completely even by condensation catalysts, and in certain cases this may adversely affect the storage stability of such organopolysiloxane resins. Thus, for example, condensation reactions of the hydroxyl groups may alter the physical properties and under certain circumstances, also the chemical properties of the products.
To solve this problem, the hydroxyl groups of the organopolysiloxane resins can be protected or specifically functionalized by subsequent reaction with suitable reagents.
The functionalization of particles which carry metal-OH and/or Si—OH groups, particularly organopolysiloxane resins, is preferably carried out using unsubstituted alkoxysilanes, such as methyltrimethoxysilane, or using functionalized and propyl-spaced alkoxysilanes, such as aminopropyltrimethoxysilane, which react under suitable reaction conditions and using catalysts, as described for example in EP 1 249 470 A and EP 1 245 627 A. In the literature a variety of catalysts such as amines and heavy metal catalysts are used, their disadvantages being mentioned in EP 564 253 A. Because of their significantly attenuated reactivity toward OH groups, propyl-spaced silanes and unfunctionalized silanes also cannot be reacted with OH-carrying particles without the addition of catalyst.
A decisive disadvantage of propyl-spaced silanes and of unfunctionalized silanes lies in the fact that monoalkoxy-functional silanes cannot generally be used for subsequent functionalization of particles, particularly of organopolysiloxane resins, since due to their low reactivity, they react extremely slowly, if at all. The use, specifically of monoalkoxy-functional silanes of this kind, would be particularly desirable for numerous applications, since the corresponding particles can be furnished extremely easily with organic functionality by means of such functionalization. Organic functionalities of this kind serve to enhance the compatibility between particles and an organic matrix in which the particles are to be used. It is particularly favorable in this context, if the organofunctionalized particles are able to react via this functionality with the matrix and so are bound chemically into the matrix. It would therefore be desirable to have organosilyl-functionalized particles that can be prepared by silanizing with monoalkoxy-functional silanes with an increased reactivity.
Particles which have been functionalized with di- and/or trialkoxysilanes may in the presence of moisture, e.g., atmospheric humidity, enter into condensation reactions with themselves or with other alkoxysilyl compounds. This occurs as a result of hydrolysis of the alkoxysilyl groups and subsequent condensation, forming Si—O—Si bonds. In this way it is possible to achieve curing of the corresponding material, on contact with atmospheric humidity, for example. The conventional silanes mentioned above, however, possess inadequate reactivity, and so this reaction is unable to occur or may occur only very slowly and in the presence of catalysts. In that case it is necessary in general to use tin catalysts, which are problematic from toxicological standpoints. Particularly low reactivity to moisture is displayed by alkoxysilane-terminated systems if, rather than methoxysilyl groups, the even less reactive ethoxysilyl groups are employed. It would therefore be desirable to have organosilyl-functionalized particles which can be prepared by silanizing with di- and/or trialkoxy-functional silanes with an increased reactivity.
WO 03/18658 and WO 03/14226 employ functionalized alkoxysilanes for the silane termination of polydimethylsiloxanes and of organic polymers, said alkoxysilanes being notable for the fact that they contain alkoxysilyl groups which are separated by a methylene spacer from a heteroatom, thereby considerably accelerating the substitution or release of alkoxy groups. The acceleration of reaction is so considerable that the fraction of catalysts required can be reduced significantly, or it is even possible to do without the addition of catalyst.